I don't know if I got something wrong here, but the docs (https://docs.gitea.io/en-us/authentication/) only explicitly list Gitea as OAuth2 Provider, while it can definitely also be a client to another provider: #1010

This initially confused me when I tried to check this capability.
And it is actually listed on the Comparison page: https://docs.gitea.io/en-us/comparison/#3rd-party-integrations